Clinical and laboratory studies were conducted to determine the etiological agents and neurological manifestations of acquired immunodeficiency syndromes in man (AIDS) and in nonhuman primates (SAIDS). Patients with neurological complications of AIDS have been admitted to the Neurology Service of the NIH Clinical Center for study. Patients admitted to other Institutes have been seen by the Infectious Diseases Branch Consultation Service. Patients have been evaluated to determine the spectrum of neurological illnesses found in AIDS. Appropriate virological and immunological studies are being conducted by IDB and collaborating laboratories. Retrovirus-like particles have been demonstrated in testes, salivary gland and prostate of patients with AIDS. Pronounced neutropenia has been detected in animals in the early stages of SAIDS with generalized immunosuppression occurring later. Isolations of SAIDS D retrovirus have been made from saliva and urine of diseased animals and SAIDS has been experimentally transmitted to a susceptible animal with an isolate from urine. Kaposi's sarcoma has also been successfully transmitted to rhesus monkeys with homogenates of tissues from animals with SAIDS. About 50% of our animals with SAIDS have developed polymyositis with many of the characteristics of viral-induced polymyositis of humans. SAIDS retrovirus has been shown to be present in and has been isolated from involved muscles. Studies of polymyositis associated with SAIDS retrovirus infections should provide a highly reproducible animal model which may further our understanding of human polymyositis. A morphological model has been developed to explain the different structures seen when AIDS and SAIDS retroviruses are thin sectioned and observed by electron microscopy. Findings from studies of nonhuman primates with SAIDS are being compared with those obtained through our AIDS protocols.